This invention relates to color-corrected photoflash lamps and units and, more particularly, to a tinted protective UV cured coating for flashlamps and color-corrected photoflash units employing such lamps.
A typical photoflash lamp comprises an hermetically sealed glass envelope, a quantity of combustible material located in the envelope, such as shredded zirconium or hafnium foil, and a combustion-supporting gas, such as oxygen, at a pressure well above one atmosphere. The lamp also includes an electrically or percussively activated primer for igniting the combustible to flash the lamp. During lamp flashing, the glass envelope is subject to severe thermal shock due to hot globules of metal oxide impinging on the walls of the lamp. As a result, cracks and crazes occur in the glass and, at higher internal pressures, containment becomes impossible. In order to reinforce the glass envelope and improve its containment capability, it has been common practice to apply a protective lacquer coating on the lamp envelope by means of a dip process. To build up the desired coating thickness, the glass envelope is generally dipped a number of times into a lacquer solution containing a solvent and a selected resin, typically cellulose acetate. After each dip, the lamp is dried to evaporate the solvent and leave the desired coating of cellulose acetate, or whatever other plastic resin is employed.
Another approach to providing a more economical and improved containment vessel is described in U.S. Pat. No. 3,893,797, wherein a thermoplastic coating, such as polycarbonate, is vacuum formed onto the exterior surface of the glass envelope.
Further approaches toward providing improved protective coatings for photoflash lamps are described in the cross-reference Dow et al and Leach et al patent applications, which relate to coatings including UV curable photopolymers. U.S. Pat. No. 4,076,489 Schroeter et al describes a dip method for applying UV cured coatings and photoflash lamps.
During ignition, photoflash lamps produce actinic radiation with a relative deficiency of shorter wavelength visible light, such as blue, and at the same time an abundance of longer wavelength visible light, such as red. Because of this, it is necessary to filter the output of the lamp to render the radiation suitable for taking photographs with color films balanced for use with daylight radiation. Selective filtration is used by those skilled in the art to accomplish proper color balance. Typically, blends of dyes and/or pigments of differing but known absorption spectra are added in various proportions to flashlamp coatings. Alternatively, other photoflash manufacturers incorporate the color correction as a tint within the cover of the photoflash unit rather than in the lamp coating. The commercial dyes added to the lamp coatings, or unit covers, for color-correcting the light output are generally organic in nature. Such dyes, or pigments, also absorb radiation in the ultraviolet region of the spectrum. Dyes used with lacquer or thermoplastic-formed coatings do not interfere with the drying, or thermal forming of the coatings, since ultraviolet radiation is not used in such processes. This is not the case, however, when organic dyes are used in conjunction with the UV cured photopolymer coatings.
Increasing the concentration and/or number of colorants in UV cured photopolymers to obtain proper color balance has an effect of absorbing more UV radiation as additional additives or concentration levels are employed. The net effect is that curing of the coating is retarded, thus weakening the coating and reducing its containment protection qualities. In order to compensate for UV radiation absorbed by the color additives, such color-corrected UV cured coatings must be irradiated for longer time periods to effect proper cure, compared to UV cured coatings which are not colored. Extension of cure time beyond that required for non-colored UV photopolymers unncessarily places limitations on production equipment suited for rapid, in-line coating automation. This, of course, adds to the cost of manufacture.
A further disadvantage is that the need for optimum UV transmission greatly limits the choice of dyes, or pigments, from which the optical filter designer can choose as additives to UV cured coatings. This unnecessarily restricts optimum design of the filter response curve to fully correct lamp emissions toward the goal of standard daylight. Furthermore, some dyes which exhibit desirable filtering characteristics in a liquid solution of UV curable photopolymers can irreversibly fade and/or change to adverse colors as a result of chemical reactions taking place during cure hardening of the photopolymer during UV exposure.
Of course, in a photoflash unit having a transparent cover disposed between the lamp and the exterior of the unit, the color-correcting dye, or pigment could be included in the cover of the unit, with the lamp coating being free of colorants. In such a situation, the optically clear and smooth appearance of the UV cured photopolymer coatings make it quite difficult to detect which flashlamps have and which have not been provided with the protective outer coating. This difficulty is particularly troublesome in the mass production manufacture of flashlamps where, because of the large numbers of lamps handled, there is a possibility of uncoated lamp envelopes becoming intermingled with coated lamps. In order to avoid this undesired possibility, it has been known in the art to alter the visual appearance of a dried coating to facilitate rapid visual inspection. For example, U.S. Pat. No. 2,781,654 Pipkin discloses a flashlamp having a lacquer coating, such as cellulose acetate, containing an ethyl silicate additive which provides a frosted or white appearance when the coating is air-dried. This additive-induced alteration of the coating appearance has the dual purpose of facilitating rapid visual quality inspection and light diffusion. Pipkin also contemplates the addition of color pigments in his coating in order to provide any desired shade or color in the frosted, light-diffusing film, such as may be desired in Christmas tree lamps. Although Pipkin provides a solution to the visual inspection problem of lacquer-coated lamps, the patent does not discuss UV curable photopolymers or the color correction of photoflash lamps or units. Further, the white or light-diffusing lacquer coating provided by the ethyl silicate additive of Pipkin is clearly not compatible with the appearance, light output, strength, and color balance requirements of the photoflash lamps and units which are the subject of the present invention, wherein the coated envelopes and covers are transparent. The lacquer coating tends to be more brittle when modified with ethyl silicate, and the resulting light-diffusing effect significantly reduces light output. Other additives, such as color pigments, in this modified lacquer coating would tend to further reduce light output. When ethyl silicate is added to the preferred UV curable photopolymer, described hereinafter (Hughson Chemical Company Type No. 3254-11), the clarity of the coating mixture is not altered even after curing by exposure to UV radiation. Hence, this additive disclosed by Pipkin is non-functional for its stated purpose in the above-mentioned UV coating for flashlamps and would detract from the coating strength otherwise obtained.